Calculate duct velocity from airflow and duct dimensions. Results populate the converter above.
Enter round duct diameter OR rectangular width and height -- not both. Airflow in CFM or L/s.
Key Conversion Factors
1 FPM = 0.00508 m/s = 0.01829 km/h = 0.01136 mph
1 km/h = 0.27778 m/s = 54.68 FPM = 0.6214 mph
Duct velocity: V (FPM) = CFM / area (ft²)
Duct velocity: V (m/s) = flow (m³/s) / area (m²)
SMACNA Recommended Duct Velocities
| Duct Location / Application | Recommended (FPM) | Recommended (m/s) | Maximum (FPM) | Maximum (m/s) |
|---|---|---|---|---|
| Residential main trunk | 600 - 900 | 3.0 - 4.6 | 1200 | 6.1 |
| Residential branch duct | 400 - 700 | 2.0 - 3.6 | 900 | 4.6 |
| Residential return main | 500 - 800 | 2.5 - 4.1 | 900 | 4.6 |
| Commercial main trunk (low vel.) | 1000 - 1500 | 5.1 - 7.6 | 2000 | 10.2 |
| Commercial branch (low vel.) | 600 - 1000 | 3.1 - 5.1 | 1500 | 7.6 |
| Commercial main trunk (high vel.) | 2000 - 3500 | 10.2 - 17.8 | 5000 | 25.4 |
| VAV box discharge | 400 - 800 | 2.0 - 4.1 | 1000 | 5.1 |
| Fan outlet velocity | 1500 - 2500 | 7.6 - 12.7 | 3500 | 17.8 |
| Outdoor air intake (louver face) | 300 - 500 | 1.5 - 2.5 | 600 | 3.1 |
| Exhaust air discharge louver | 500 - 800 | 2.5 - 4.1 | 1000 | 5.1 |
Terminal Device Face Velocities
| Device | Typical (FPM) | Typical (m/s) | Max for Low Noise | Notes |
|---|---|---|---|---|
| Supply ceiling diffuser (neck) | 400 - 600 | 2.0 - 3.1 | 700 FPM / 3.6 m/s | NC 25-35 for offices |
| Supply linear diffuser (face) | 300 - 500 | 1.5 - 2.5 | 600 FPM / 3.1 m/s | Perimeter heating / cooling |
| Supply floor diffuser | 200 - 350 | 1.0 - 1.8 | 400 FPM / 2.0 m/s | Underfloor air distribution |
| Return air grille (face) | 400 - 600 | 2.0 - 3.1 | 700 FPM / 3.6 m/s | Higher acceptable away from occupants |
| Transfer air grille | 200 - 400 | 1.0 - 2.0 | 500 FPM / 2.5 m/s | Room-to-room pressure relief |
| Exhaust grille (bathroom) | 400 - 700 | 2.0 - 3.6 | 900 FPM / 4.6 m/s | Higher velocity acceptable in bathrooms |
| Kitchen exhaust hood (face) | 100 - 150 | 0.5 - 0.75 | 200 FPM / 1.0 m/s | Capture velocity at hood face |
| Throw terminal velocity (ASHRAE 55) | 50 | 0.25 | 50 FPM / 0.25 m/s | Max at occupied zone boundary |
Face velocities are at rated flow. Actual selection should use manufacturer performance data and NC curves. Use the Duct Sizing Calculator for full system velocity design.
How to Use the Air Velocity Converter
Enter a velocity in FPM, m/s, km/h, mph, or ft/s. All other units update instantly. Use the quick-reference presets for common HVAC duct velocities: residential branch, main trunk, commercial, and VAV systems. The context band identifies what HVAC application your velocity corresponds to.
Use the Duct Velocity Calculator to compute velocity from airflow rate and duct dimensions. Enter CFM or L/s, then either the round duct diameter in inches or the rectangular duct width and height. Results show velocity in FPM and m/s and populate the unit converter above automatically.
The SMACNA velocity reference table shows recommended and maximum velocities for each duct location. Compare your calculated or converted velocity against these limits to confirm the duct is sized for acceptable noise, pressure drop, and energy use. Residential main trunks should stay under 900-1200 FPM (4.6-6.1 m/s).
The terminal device table covers supply diffusers, return grilles, exhaust fans, and kitchen hoods. Confirm that your diffuser neck velocity and throw terminal velocity stay within comfort limits per ASHRAE 55. Feed duct sizes and velocities into the Duct Sizing Calculator for a complete system design.
Air Velocity in HVAC Duct Design -- Complete Guide
Air velocity is central to duct system performance. Too high and you get noise, high static pressure, and wasted fan energy. Too low and you need oversized ductwork. Canadian HVAC engineering uses m/s while North American equipment and SMACNA design references use FPM. Converting fluently between the two is a daily requirement in duct design and commissioning work.
FPM to m/s -- The Core Conversion
The conversion factor is exact: 1 FPM = 0.00508 m/s, or 1 m/s = 196.85 FPM. For quick field estimates, 200 FPM is close to 1 m/s, and 1000 FPM is close to 5 m/s. SMACNA duct design tables use FPM; Canadian engineering drawings and commissioning reports use m/s. A residential main trunk velocity of 750 FPM equals 3.81 m/s -- well within the recommended 600-900 FPM (3.0-4.6 m/s) range. Use the Airflow Unit Converter to convert CFM to L/s before feeding into metric velocity calculations.
Duct Velocity and Sizing
Duct velocity equals airflow divided by cross-sectional area: V = Q / A. In Imperial: FPM = CFM / ft². In metric: m/s = m³/s / m². A 10-inch round duct (area = 0.5454 ft²) carrying 400 CFM has a velocity of 400 / 0.5454 = 734 FPM (3.73 m/s). That's within the SMACNA residential branch range of 400-700 FPM -- slightly high, suggesting a 12-inch duct (area = 0.7854 ft², velocity = 509 FPM / 2.59 m/s) would be a better choice. Use the duct velocity calculator above to check any duct quickly, or the Duct Sizing Calculator to size a full duct system.
Velocity and Noise in Occupied Spaces
Duct-generated noise is roughly proportional to the fifth power of velocity -- doubling the velocity increases noise by about 15 dB. This is why keeping residential branch velocities under 700 FPM (3.6 m/s) and supply diffuser neck velocities under 600-700 FPM (3.1-3.6 m/s) matters so much for occupant comfort. The ASHRAE 55 comfort standard limits occupied zone air speed to 0.2-0.25 m/s (40-50 FPM) at head height for sedentary occupants. The jet from a supply diffuser must decelerate from 400-600 FPM at the neck to under 50 FPM before reaching the occupied zone. Diffuser throw ratings from manufacturers show the distance to the 50 FPM terminal velocity point.
High-Velocity Systems and VAV
Commercial high-velocity systems use 2000-5000 FPM (10.2-25.4 m/s) in main duct to reduce duct sizes and installation cost, then reduce velocity at VAV boxes and downstream branches. VAV box discharge should stay under 800-1000 FPM (4.1-5.1 m/s) to avoid excessive turbulence and noise at partial load. The pressure drop at high velocities increases sharply -- pressure drop scales with velocity squared. A duct at 2000 FPM has 4x the pressure drop per unit length of the same duct at 1000 FPM. For high-velocity system design, use the Pressure Converter alongside this velocity tool.
Frequently Asked Questions
Multiply FPM by 0.00508 to get m/s. So 500 FPM = 2.54 m/s, 1000 FPM = 5.08 m/s, and 2000 FPM = 10.16 m/s. To go the other way, multiply m/s by 196.85. A quick mental shortcut: 200 FPM is approximately 1 m/s. This conversion is needed whenever SMACNA duct design tables (FPM) are used alongside Canadian engineering drawings (m/s). The calculator above handles the full five-unit conversion in real time.
SMACNA recommends residential main trunk velocities of 600-900 FPM (3.0-4.6 m/s) and branch duct velocities of 400-700 FPM (2.0-3.6 m/s). Return main ducts should run at 500-800 FPM (2.5-4.1 m/s). Exceeding these limits increases noise and static pressure, which reduces airflow delivery and increases fan energy. Supply diffuser neck velocities should stay under 600-700 FPM (3.1-3.6 m/s) to avoid draft and noise complaints in the occupied space.
Supply ceiling diffusers in occupied spaces should not exceed 600-700 FPM (3.1-3.6 m/s) at the neck to stay within NC 30-35 for offices and NC 25-30 for quiet spaces. The throw terminal velocity -- the air speed at the edge of the diffuser's pattern -- must decay to 50 FPM (0.25 m/s) before reaching the occupied zone per ASHRAE 55. Always use manufacturer throw tables and NC rating curves to verify diffuser selection at your actual CFM, not just the face velocity limit.
Duct velocity = airflow / cross-sectional area. In Imperial: velocity (FPM) = CFM / area (ft²). In metric: velocity (m/s) = flow rate (m³/s) / area (m²). For a 10-inch round duct, area = pi x (5/12)² = 0.5454 ft². At 400 CFM: 400 / 0.5454 = 734 FPM (3.73 m/s). Use the Duct Velocity Calculator on this page to compute instantly, or the Duct Sizing Calculator to work through a full duct system with static pressure calculations.